Valve assembly of this kind, where the spring seat is disposed slidably around the supporting member, is known from the state of art (cf. for example Polish patent applications P. 358597, P. 358598, P. 358599 and European patent applications EP 1925845 A1, EP 2233775). For its efficient operation it is absolutely essential to ensure substantially frictionless sliding (axial) movement of the spring seat with respect to the supporting member.
Various factors contribute to this problem including eccentricity and radial dimensional tolerances of particular elements of the valve assembly, possible buckling of the spring and distribution of the axial pressure exerted by the spring over the perimeter of the spring seat.
Influence of friction on operation of a valve assembly of this kind may be observed by measuring the force reaction of a damper in dependence to the velocity of the piston rod (a force to velocity characteristic). As it appears a substantial hysteresis occurs.
In other words forces measured while the piston rod accelerates to trigger opening of the spring valve assembly and deflection of discs are higher than forces measured while the piston rod decelerates to close the spring valve assembly and discs move to their neutral position. A difference exists therefore between damping forces generated by the damper for the same velocity of the piston rod but measured during its acceleration or deceleration and it is desirable to limit this difference as much as possible in order to improve the damper performance, repeatability coefficient and also the comfort for the passengers of the vehicle.
Patent specification GB 699896 discloses a hydraulic shock absorber, where a spring seat has a form of an abutment ring having an outwardly extending flange which rests upon the outer deflectable disc adjacent its outer edge. The outer deflectable disc is provided with an orifice covered by the inner deflectable disc and a clearance exists between the abutment ring and a cylindrical member that allows fluid communication through said orifice between the flow channels and the chamber of the shock absorber once the discs are deflected.
Patent specification U.S. Pat. No. 4,096,928 discloses a valve assembly for a shock absorber, where a spring seat has a form of a valve cage having a cylindrical portion surrounding the resilient valve disc for a centered guiding thereof. The cylindrical portion has an edge face which serves as an abutment for cooperating with the closing disc.
It has been the object of the present invention to provide a hydraulic damper having an improved sensitivity of operation in a result of minimizing the friction between cooperating elements of the valve assembly.
Another object of the present invention is to provide a hydraulic damper that would enable to achieve comparable working characteristic of all dampers in the line production within a large range of dimensional tolerances of the damper components in order to minimize the production specific losses, decrease the costs of production and increase the result repeatability coefficient.